In handling and processing of bulk solids such as sugar, coal, drink mixes, wood chips, and various plastics (typically pellets) in a bin or hopper, it often becomes essential that the solids velocity profile across any section of the bin and/or hopper be uniform so that the bin or hopper produces a first-in/first-out flow sequence. When this flow sequence is achieved, then processing requiring gas or liquid counterflow or concurrent flow have the opportunity to maximize their efficiency. Problems of segregation of different particle sizes can be substantially eliminated and interfaces between batches of product in the same bin can be clearly defined. Achieving such a result requires that the feeder and the hopper act in unison to produce the flow uniformity desired. The larger the hopper outlet, the more sensitive flow uniformity is to the feeder design. The present invention addresses the design of a screw feeder under a slot and the associated hopper to produce the uniform flow velocity distribution desired.
Prior screw feeder art has focused mostly on compaction or extrusion screws (e.g. U.S. Pat. No. 5,052,874). No attempt has been made to integrate the workings of the screw and the hopper to control solids flow patterns. As a result, the flow from a hopper or bin is somewhat unpredictable and not suited to fully effective solids processing.
According to one aspect of the present invention a bin or hopper discharge assembly is provided comprising the following components: A bin or hopper having at least one bottom slot outlet of effective length L. At least one screw in a trough and rotatable with a screw shaft of diameter d and having flights with a flight diameter f, and an average pitch P and an effective length substantially equal to L, the screw receiving material flowing through the at least one slot outlet, and having at least one (preferably only one) discharge from the screw trough. The screw having a varying average pitch, shaft diameter, and flight diameter, and an effective length L; and the screw constructed to correspond to the formula ##EQU1##
where f.sub.n, d.sub.n, and P.sub.n are the flight diameter, shaft diameter and pitch of the screw at the nth pitch increment along the length L, and f.sub.L, d.sub.L, and P.sub.L are the flight diameter, shaft diameter, and pitch of the last or final pitch increment under the flow outlet near the screw discharge. The bin or hopper preferably is one having one-dimensional convergence.
The assembly preferably further comprises at least one flow rate adjuster mechanism which provides flow regulation of material transported by the screw. For example, the adjuster mechanism may comprise a substantially vertical plate with a substantially straight-across bottom curved about a substantially horizontal axis. Alternatively, the adjuster mechanism may comprise a substantially vertical plate with a bottom having two-dimensional curvature. In both cases the plate's axial positions are adjustable over a part of the length of the screw, and also may be vertically adjustable. The flow rate adjusters may also be used with screws having uniform pitch, shaft diameter and flight diameter.
The at least one screw may consist of a single screw, and the at least one slot outlet may consist of a single slot outlet, and the at least one discharge may consist of a single discharge. The screw typically has front and back ends, and the at least one flow rate adjustment mechanism may comprise an adjustment mechanism at each of the front and back ends.
Alternatively, the at least one slot outlet may comprise first and second slot outlets, the at least one screw comprises a right hand screw cooperating with the first slot outlet and a left hand screw cooperating with the second slot outlet, the screws on a common shaft. In this case the at least one discharge may comprise a single common protected discharge between the first and second slot outlets, and a hanger bearing may be provided for the shaft adjacent the protected discharge. The first and second slot outlets are preferably in intermediate one-dimensional convergence hoppers beneath a larger long slot outlet hopper with one-dimensional convergence. The flow rate adjuster mechanism may be mounted adjacent an end of each screw opposite the common discharge.
Alternatively, the at least one screw consists of a single screw, and wherein the at least one slot outlet comprises more than two slot outlets from more than two intermediate one-dimensional convergence hoppers beneath a larger long slot outlet hopper with one-dimensional convergence, the more than two slot outlets collectively having the effective length L, the more than two slot outlets each cooperating with the single screw. Preferably, the at least one discharge comprises a single discharge, and a flow rate adjuster mechanism is associated with each of the more than two slot outlets. Typically, the bin or hopper contains, and feeds with a substantially uniform flow velocity distribution along the slot outlet, coal, wood chips, plastic pellets, sugar, or drink mixes, or like bulk solid particulate material.
According to another aspect of the present invention a bin or hopper assembly is provided comprising: A bin or hopper having at least one bottom slot outlet of effective length L. At least one screw in a trough for receiving material from the at least one slot outlet and having an effective length substantially equal to L. The at least one screw rotatable with a screw shaft having a diameter, and having flights with a flight diameter and an average pitch. A single discharge from the at least one screw trough. And at least one flow rate adjuster mechanism which provides flow regulation of material transported by the screw. The screw shaft diameter, flight diameter, and average pitch preferably varying along the shaft length so as to provide a substantially uniform material flow velocity for distribution along the slot outlet. The bin or hopper preferably has one-dimensional convergence.
According to yet another aspect of the present invention a bin or hopper discharge assembly is provided comprising: A bin or hopper having at least one bottom slot outlet of effective length L. At least one screw in a trough and rotatable with a screw shaft and having flights, and an average pitch, and an effective length substantially equal to L, the screw receiving material flowing from the at least one slot outlet. At least one discharge from the screw trough. And the screw having a varying average pitch, shaft diameter, and flight diameter, and an effective length L, providing a substantially uniform flow velocity distribution along the slot outlet of solid particulate material flowing from the bin or hopper to the screw discharge. The bin or hopper preferably has one-dimensional convergence.
According to still another aspect of the invention an assembly is provided comprising: A bin or hopper (preferably having one-dimensional convergence) having at least one bottom slot outlet of effective length L. At least one screw in a trough for receiving material from the at least one slot outlet and having an effective length substantially equal to L. The at least one screw rotatable with a screw shaft having a diameter, and having flights with a flight diameter, and an average pitch. A single discharge from the at least one screw trough. The screw having a varying average pitch, shaft diameter, and flight diameter along the length thereof, and wherein the screw varying average pitch, shaft diameter, and flight diameter provide a substantially uniform flow velocity distribution along the slot outlet of solid particulate material flowing from the bin or hopper to the screw discharge.
It is a primary object of the present invention to provide enhanced uniformity of flow velocity distribution from a hopper or bin using a particular screw feeder. This and other objects of the invention will become clear from an inspection of the detailed description of the invention and from the appended claims.